The invention relates to an optical semiconductor device with a multiple quantum well structure, in which well layers and barrier layers comprising various types of semiconductor layers are alternately layered.
A device of this type is known for example from EP 0 666 624 B1 or from the Journal of Crystal Growth 189/190 (1998) pages 786-789.
The high quantum efficiency of indium-gallium-nitride (InGaN)-based LEDs and laser diodes is caused by the self-organized growth of indium-rich islands in the active quantum well. As a result, the injected charge carriers are spatially localized at these islands and are prevented from non-radiating recombination at lattice defects.
The nucleation of these quantum dots must be initiated by suitable buffer layers. In particular, indium-containing structures are suitable before the actual active zone as a buffer layer. Indium-containing nitridic semiconductors (GaxAlyInl-(x+y)N semiconductors) have a tendency toward segregation and formation of indium-containing phases. This leads to varying strain fields at the growth surface, promoting the formation of indium-rich islands in the active quantum well. GaInN layers approximately 100 nm thick can be deposited before the active zone in order to improve the GaInN quantum dot nucleation.
Previously, an optimum efficiency could be achieved with, for example, 2- to 10-fold quantum well structures. As can be shown experimentally, the emitted photons are generated exclusively in the two uppermost quantum wells (i.e. those closest to the p side). A suitable choice of growth parameters achieves the effect that the emission takes place exclusively in the uppermost of the quantum wells. The lower quantum wells serve for improving the nucleation of the GaInN islands in the uppermost quantum well. Dispensing with them causes the optical output power to be reduced by over 50%. However, these quantum wells lead to a considerable increase in the forward voltage. The forward voltage can be improved by reducing the number of wells at the expense of the quantum efficiency. The piezo fields, which lead to the observed increase in the forward voltage, can be compensated by high doping levels in the quantum well region. However, this adversely effects the crystal quality of the active layer or impairs the injection behavior and consequently reduces the internal quantum efficiency.